Pneumatic tire for motorcycle

ABSTRACT

This invention relates to a pneumatic tire for a motorcycle improving a cornering stability, and more particularly to a pneumatic tire for a motorcycle comprising a pair of bead portions  2 , a carcass  1  wound around a bead core  3  embedded in the respective bead portion, a tread portion arranged on an outside of a crown portion of the carcass and a pair of sidewall portions extending inward from both ends of the tread portion in a radial direction of the tire, in which the bead core  3  is a cable bead and a geometric second moment coefficient Σ(π/64×d 4 ) of the bead core  3  (wherein d is a diameter of each filament constituting the bead core) satisfies an equation of 0.50&lt;Σ(π/64×d 4 )&lt;1.50.

TECHNICAL FIELD

This invention relates to a pneumatic tire for a motorcycle (hereinafterreferred to as “motorcycle tire” or “tire” simply), and moreparticularly to a pneumatic tire for a motorcycle in which the runningstability is improved while enhancing the durability by improving a beadcore.

BACKGROUND ART

The motorcycle has a great feature different from a four-wheeled vehiclein a point that the vehicle body is largely inclined in the running on acorner. For this end, the tire applied to the motorcycle is required tobe excellent in the running stability during the cornering. As atechnique for ensuring the running stability during the cornering, it iseffective to improve the ground contacting property of the tire. From aviewpoint of the improvement of the ground contacting property, it ispreferable that the deformation of not only the tread rubber for thetire but also the carcass inside the tire is easy.

As a way for facilitating the deformation of the carcass, it iseffective that the deformation of a carcass ply wound around a bead coreis allowed to a certain level in an axial direction of the tire. Forthis end, it is usually required to lower the distortion rigidity of thebead core. If it is intended to realize this requirement by decreasingthe winding number of the wire, the fracture strength of the beadportion lowers. In the motorcycle tire, it is difficult tosimultaneously establish the ground contacting property of the tire andthe safety of the bead portion. Therefore, it is desired to develop atechnique capable of sufficiently ensuring the running stability duringthe cornering.

On the other hand, JP-A-2002-254910 discloses a technique that thedeformation of the carcass ply around the bead core can be allowedwithout lowering the distortion rigidity of the bead core by arrangingan innerliner rubber layer enclosing the bead core along the carcass plybetween the carcass ply and the bead core to render a compression ratioof the bead portion into a predetermined range.

In the tires for passenger car, so-called cable bead is applied to thebead core for reinforcing the bead portion to realize the stable runningproperty (see JP-A-06-21100). The cable bead means a bead core having astructure that one or more sheath layers are helically wound around asingle welded core wire. However, a cable bead comprising two or moresheath layers has not hitherto been used in the motorcycle tire becausea region capable of arranging the bead core is narrower. An example ofapplying a cable bead comprising one sheath layer around the core wireto the motorcycle tire is disclosed, for example in JP-A-2002-254910.

DISCLOSURE OF THE INVENTION

As mentioned above, it is demanded in the pneumatic tire for themotorcycle to develop a technique that the running stability in thecornering can be improved by simultaneously establishing the improvementof the ground contacting property and the improvement of the safety inthe bead portion. It is, therefore, an object of the invention toprovide a pneumatic tire for a motorcycle in which the bead core isimproved to balance the ground contacting property in the cornering andthe safety in the bead portion to thereby improve the corneringstability.

The inventors have made various studies for solving the above problemsand found that it is effective to define a geometric second momentcoefficient of the bead core to a predetermined range for attaining theestablishment of the ground contacting property and the strength of thebead portion, and as a result, the invention has been accomplished.

According to the invention, there is the provision of a pneumatic tirefor a motorcycle comprising a pair of bead portions, a carcass woundaround a bead core embedded in the respective bead portion, a treadportion arranged on an outside of a crown portion of the carcass and apair of sidewall portions extending inward from both ends of the treadportion in a radial direction of the tire, in which the bead core is acable bead and a geometric second moment coefficient Σ(π/64×d⁴) of thebead core (wherein d is a diameter of each filament constituting thebead core) satisfies an equation of 0.50<Σ(π/64×d⁴)<1.50.

In the invention, it is preferable that the bead core is a multi-layercable bead, particularly two-layer cable bead.

According to the above construction of the invention, the carcass iseasily moved in the axial direction of the tire to improve the groundcontacting property under a camber load in the cornering, while thesafety of the bead portion can be ensured within an adequate range toenhance the safety, whereby it is made possible to realize a pneumatictire for a motorcycle improving the cornering stability without loweringthe safety of the bead core.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged section view showing a vicinity of a bead portionin the pneumatic tire for the motorcycle according to the invention;

FIG. 2 is an enlarged perspective view showing an embodiment of atwo-layer cable bead;

FIG. 3 is a graph showing a relation between a diameter of a wire and atensile strength; and

FIG. 4 is an enlarged section view showing a structure of a bead core(cable bead) of each of Examples 1-3 and Comparative Examples 1-3.

BEST MODE FOR CARRYING OUT THE INVENTION

A best mode of an embodiment of the invention will be concretelyexplained with reference to the drawings below. As shown in FIG. 1, thepneumatic tire for the motorcycle according to the invention comprises apair of bead portions 2, a carcass 1 wound around a bead core 3 embeddedin the respective bead portion, a tread portion arranged on an outsideof a crown portion of the carcass but not shown, and a pair of sidewallportions extending inward from both ends of the tread portion in aradial direction of the tire. In the invention, it is important that thebead core 3 is a cable bead and a geometric second moment coefficientΣ(π/64×d⁴) thereof satisfies an equation of 0.50<Σ(π/64×d⁴)<1.50.

The geometric second moment coefficient Σ(π/64×d⁴) of the bead core 3means a sum of geometric second moment coefficients of filaments arounda core constituting the bead core 3, and d is a diameter of eachfilament around the core constituting the bead core 3. When the beadcore 3 is the cable bead and has the geometric second moment coefficientof the above range, the ground contacting property can be enhancedwithout damaging the safety of the bead portion and the good corneringstability can be obtained.

In the invention, the concrete structure, material and the like of eachmember of the tire other than the bead core are not particularly limitedas far as the geometric second moment coefficient of the bead core 3using the cable bead satisfies the above range. Preferably, the beadcore 3 is a multi-layer cable bead. As previously mentioned, themulti-layer cable bead comprising two or more sheath layers has nothitherto been used as a bead core in the motorcycle tire. On thecontrary, the inventors have found that the multi-layer cable bead canbe preferably used in the motorcycle tire according to the inventionfrom the following reasons.

As seen from a graph of FIG. 3 showing a relation between filamentdiameter and tensile strength, as the diameter of the wire filamentprepared from the same material is made thinner by drawing, the strength(tensile strength) per unit sectional area becomes generally higher.Also, it is considered that the strength utilizing ratio in the sheathlayer is excellent in the filaments of the multi-sheath layers wound soas to cross with each other as compared with the filaments of the singlesheath layer wound around the core wire in one direction because stressacting to the whole of the bead portion is dispersed uniformly. Owing tothese two factors, the multi-layer cable bead is possible to enhance thesafety at the same weight as compared with the single layer cable bead,while even at the same safety, it is possible to reduce the weight ofthe bead core. In the motorcycle tire, therefore, it is considered thatit is possible to increase the strength of the bead portion moreefficiently by using the multi-layer cable bead.

Also, the cable bead has a ring shape formed by helically winding thewire filaments as previously mentioned, so that it is possible tosmoothen the carcass centering on the axial center of the bead core ascompared with the usual bead core having a polygonal shape in section.That is, the effect of improving the ground contacting property can alsobe obtained by using the cable bead as a bead core.

As the multi-layer cable bead is preferable a two-layer cable beadthough the number of sheath layers may be two or more. Because, when thediameter of the wire filament is fine to an extent required for theformation of three sheath layers, the bending rigidity of the bead corebecomes extremely low and hence the disorder of the filaments becomeslarge in the shaping step of the bead core or the production step of thetire and there is a tendency of decreasing the strength utilizing ratio.

In FIG. 2 is shown an example of the two-layer bead cable. Theillustrated two-layer bead cable 10 is formed by sequentially winding afirst sheath layer 12 and a second sheath layer 13 around a core wire11. The number of filaments and filament diameter of each sheath layerin the multi-layer cable bead are not particularly limited as far as thegeometric second moment coefficient of the bead core satisfies theabove-defined range. In case of the illustrated two-layer cable bead 10,the number of filaments in the first sheath layer 12 is usually 5-7 andthat in the second sheath layer 13 is about 10-16. Also, the filamentdiameter is 1.0-1.5 mm in the core wire 11, 0.9-1.4 mm in the firstsheath layer 12, and 0.9-1.4 mm in the second sheath layer 13.

The invention will be described concretely with reference to thefollowing examples below.

EXAMPLES 1-3 AND COMPARATIVE EXAMPLES 1-3

There are prepared pneumatic tires for motorcycle having a tire size of120/70R17 by using a structure of a bead core (cable bead) shown inTable 1 and FIGS. 4( a)-(f). With respect to the thus obtained tires,the geometric second moment coefficient Σ(π/64×d⁴) of the bead core ismeasured and the following test for the stability in the cornering ismade to obtain results as shown in Table 1.

The test for the stability in the cornering is carried out by assemblingeach test tire into a rim of MT3.5×17 and adjusting an air pressure to200 kPa and mounting on a rear wheel of a motorcycle having adisplacement of 125 cc to conduct a feeling evaluation. Concretely, thetire of each of the examples and comparative examples is mounted on therear wheel under a condition that a tire for front wheel is the same andrun at a speed of 80 km/h on a cornering course to evaluate a steadystate of rigidity as an indication of stability in cornering by adriver's feeling, which is represented by an index on the basis that thetire of Comparative Example 1 is 100.

TABLE 1 Compar- Compar- Compar- ative ative ative Example ExampleExample Example Example Example 1 2 1 2 3 3 Structure Diameter of core2.20 2.40 1.20 1.20 1.20 0.8 of bead wire (mm) core First Diameter 1.401.60 0.96 0.96 1.20 0.7 sheath (mm) layer Number of 8   8   6   6   6  6   Filaments Second Diameter — — 0.96 1.20 0.96 0.7 sheath (mm) layerNumber of — — 12   11   14   13   Filaments Third Diameter — — — — — 0.7sheath (mm) layer Number of — — — — — 20 Filaments Section view (a) (b)(c) (d) (e) (f) (FIG. 4) Geometric second 1.51 2.57 0.75 1.37 1.19 0.46moment coefficient Stability in 100    90    110    120    115    90   cornering (index)

INDUSTRIAL APPLICABILITY

As mentioned above, according to the invention, there can be realized apneumatic tire for a motorcycle in which the cornering stability can beimproved by improving the bead core to balance the ground contactingproperty in the cornering and the safety of the bead portion.

1. A pneumatic tire for a motorcycle comprising a pair of bead portions,a carcass wound around a bead core embedded in the respective beadportion, a tread portion arranged on an outside of a crown portion ofthe carcass and a pair of sidewall portions extending inward from bothends of the tread portion in a radial direction of the tire, in whichthe bead core is a cable bead and a geometric second moment coefficientΣ(π/64×d⁴) of the bead core (wherein d is a diameter of each filamentconstituting the bead core except for a core wire) satisfies an equationof 0.50<Σ(π/64×d⁴)<1.50, wherein the bead core is a multi-layer cablebead.
 2. A pneumatic tire for a motorcycle according to claim 1, whereinthe bead core is a two-layer cable bead.